Indirectly heated cathode-heater assembly and support means therefor

ABSTRACT

Presented is a cathode structure which is planar in form and which may be scaled up or down to meet the needs of the particular user. The cathode is constructed so as to present a rugged flat package which may easily be mounted on an associated support structure.

United States Patent Barraco et al.

[ 5] Sept. 16, 1975 INDIRECTLY HEATED CATHODE-HEATER ASSEMBLY AND SUPPORT MEANS THEREFOR Inventors: Anthony J. Barraco, San Jose;

Albert M. Rockwell, Jr., Belmont, both of Calif.

Assignee: Anthony J. Barraco, San Jose, Calif.

Filed: Jan. 18, 1974 Appl. No.: 434,574

US. Cl. 313/337; 313/278; 313/346; 313/340 Int. Cl. HOIJ l/20; HOlJ 19/14 Field of Search 313/337, 346, 310, 260, 313/278, 287, 340

References Cited UNITED STATES PATENTS Brian 313/337 X 2,899,591 8/1959 Stein 313/337 3,175,118 3/1965 Ney.... 313/337 X 3,304,459 2/1967 Shaw 313/337 3,439,210 4/1969 Gallagher et a1. 313/337 3,440,475 4/1969 Schiller et a1. 313/337 3,465,195 9/1969 Fuchs 313/337 X Primary Examiner-Saxfield Chatmon, Jr. Attorney, Agent, or Firm-John J. Leavitt [57] ABSTRACT Presented is a cathode structure which is planar in form and which may be scaled up or down to meet the needs of the particular user. The cathode is constructed so as to present a rugged flat package which may easily be mounted on an associated support structure.

53 Claims, 12 Drawing Figures PATENTEUSEPIESISYS 3,906,276

sum 3 0f 3 INDIRECTLY HEATED CATHODE-HEATER ASSEMBLY AND SUPPORT MEANS THEREFOR BACKGROUND OF THE INVENTION With the increasing premium placed on space in electron discharge devices, it is an advantage to construct a planar cathode in an assembly that is as flat as can be and still perform its intended function. Heretofore, most cathode structures utilized a cylindrical cathode symmetrical about a central axis and on the can closed end of which is mounted the emissive coating from which the electrons emanate. The tubular can that supports the cathode coating is utilized to house an elongated heater element which, when energized, heats the cathode structure to the cathode operating temperature. Various ways and means have been devised to support such an elongated cathode can in proper relationship to other elements in the discharge device so as to retain the cathode in proper position. Such conventional supporting structures always pose problems in that to effect a rigid support for the cathode can it is necessary that the supporting devices be sufficiently massive to perform that function. But it is found that the very massivness of the support structure detracts from the efficiency of the cathode in that it constitutes a large conductive and radiative path by which heat may escape from the cathode assembly, thus requiring greater power in the cathode heater to bring the cathode coating to the temperature necessary for a satisfactory state of electron emissivity. Accordingly, it is one of the objects of the present invention to provide a cathode construction that is planar in its configuration and which is supported in a manner to minimize the flow of heat away from the cathode.

Cathode assembly construction has always been relatively expensive, particularly for those type of cathodes that must be individually fabricated, or attended to in one way or another, such as by the insertion of an insulated coil heater and associated electrical connection points and heater supports. Accordingly, it is another object of the present invention to provide a cathode structure which is susceptible to mass production so as to minimize the cost of manufacture and therefore the ultimate cost of the cathode to the consumer.

Another disadvantage of conventional cathode construction has been the tendency toward fragility of the cathode assembly. Thus, impact shocks on an electron gun or other discharge device utilizing a conventional cathode assembly will frequently cause sufficient displacement of the cathode that it impairs use of the electron gun or discharge device. Most such conventional cathode structures are particularly susceptible to impact shocks that include a laterally or transversely directed shock wave. Accordingly, it is another object of the invention to provide a cathode structure that is especially resistant to being displaced by a transversely imposed shock or impact wave.

In an attempt to minimize the loss of heat, and therefore increase the efficiency of conventional cathodes, it has been the practice to surround or associate conventional cathodes with different types of thermally reflective surfaces that will tend to reflect to the working surface of the cathode whatever heat is radiated from the heater or cathode assembly. The present conventional application of such reflective devices or surfaces in a cathode assembly causes the assembly to be unnecessarily complex and large with respect to the area of the emitting surface and the function to be performed by the cathode itself. It is therefore a still further object of the present invention to provide a cathode construction in which the heat generated by the heater unit associated with the electron emissive surface of the cathode is inherently channeled toward the electron emissive surface of the cathode for greater efficiency.

Another object of the invention is to provide a cathode-heater assembly in which the thermal coupling between the heater and cathode is enhanced so that for a given cathode operating temperature, the heater temperature is lower, leading to longer heater life.

A frequent disadvantage that is encountered with a conventional cathode utilizing an elongated coil heater enclosed within an elongated tubular cathode can is the fragility of the coil heater per se. Such heater packages are generally fabricated from very fragile filaments which tend to be brittle and therefore susceptible to being broken by shocks imposed on the cathode assembly during the manufacture or subsequent use. They also tend to deform with age thus increasing thermal impedance between the heater and cathode, changing cathode temperature and reducing life. Attempts have been made to support the coil heater by embedding it in various types of materials, but such efforts are costly to produce, leading to increased cost to the ultimate consumer. Accordingly, it is another object of the present invention to provide a cathode assembly including a cathode unit and a heater unit for such cathode, both of which are planar and in which the heater unit is rugged and capable of being mass produced, and remains in close reproduceable, and relatively constant thermal proximity to the cathode.

Still another object of the invention is to provide a cathode-heater assembly of extremely rugged construction in which the heater is planar and lies parallel to the emissive coating of the cathode, with the heater being embedded in such a way that it is mechanically strengthened and thermally effective to heat the electron emissive surface of the cathode.

A still further object of the invention is the provision of a cathode-heater assembly in which the relationship of the heater to the cathode is maintained constant over the life of the cathode.

Still another object of the invention is the provision of a planar cathode which may be fabricated in modular sub-assemblies which may then be assembled to produce the finished planar cathode.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be apparent from the following description and the drawings. It is to be understood, however, that the invention is not limited to the embodiment illustrated and described, since it may be embodied in various forms within the scope of the appended claims.

SUMMARY OF THE INVENTION In terms of broad inclusion, the cathode structure of the invention includes two sub-assemblies, one of which may be designated the cathode assembly, and the other of which may be designated the support struc ture therefore. The cathode sub-assembly includes a flatmetallic surface on or in which the electron emissive materials forming the emissive layer of the cathode are supported, and adjacent thereto is a heater assembly which also possesses a flat configuration and which is related to the associated surface carrying the emis sive layer in such a manner that maximum thermal efficiency is derived from the heater assembly. Leads forming a part of the heater assembly extend away from the heater and join with the support sub-assembly to rigidly support the cathode assembly in relation to the support structure. The support structure is conveniently annular in its configuration and forms the means by which the cathode assembly may be mounted in its ultimate environment. Means are provided on the support sub-assembly for tensioning the heater legs so that retention of the cathode package including the heater in proper position during thermal cycling and operation is assured. Means are also provided to permit electrical connections to the heater and cathode through relatively massive tabs which are anchored in the support structure so attachment of and stresses in the connecting leads do not induce stresses in the cathode support leads.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of the planar cathode of the invention, portions of the structure being broken away to reveal the underlying parts.

FIG. 2 is a vertical cross-sectional view taken in the plane indicated by the line 22 in FIG. 1.

FIG. 3 is an enlarged fragmentary view in plan of one form of heater unit that may be used in conjunction with the cathode package.

FIG. 4 is an enlarged fragmentary view of the cathode package, a portion of the heater legs being broken away to reduce the size of the view, and parts of the structure being shown in section for clarity of illustration.

FIG. 5 is an enlarged fragmentary cross-sectional view taken in the plane indicated by the line 55 in FIG. 4.

FIG. 6 is an enlarged fragmentary cross-sectional view of a slightly different embodiment of heater structure.

FIG. 7 is an enlarged fragmentary cross-sectional view illustrating still another embodiment of heater structure.

FIG. 8 is an enlarged fragmentary view, partly in vertical section, through a peripheral portion of the support sub-assembly.

' FIG. 9 is an enlarged fragmentary view in plan of another embodiment of the heater unit.

FIG. 10 is a fragmentary plan view of a modified cathode assembly and support structure in which the reflector is electrically isolated from the cathode and in which the support sub-assembly is modified somewhat.

FIG. 11 is an enlarged fragmentary cross-sectional view of a portion of the embodiment illustrated in FIG. 10.

FIG. 12 is an enlarged fragmentary plan view of another embodiment of the cathode-heater sub-assembly showing how the cathode may be rigidly mounted yet electrically isolated from the heater.

DESCRIPTION OF TH PREFERRED EMBODIMENTS In terms of greater detail, the planar cathode of the invention in one aspect is illustrated in FIG. 1 in plan, and as there shown, the cathode structure includes a cathode assembly designated generally by the numeral 2 and a support assembly for the cathode assembly designated generally by the numeral 3. In the interest of brevity, the cathode assembly will be described as a sub-assembly and then the support structure for the cathode assembly will be described as a second subassembly, and then the two sub-assemblies will be described as a combination cooperating one with the other to produce the planar cathode structure of the invention. Different embodiments of the preferred form will be described where appropriate.

CATHODE ASSEMBLY Referring to FIG. 1, it will there be seen that the cathode assembly 2 includes a flat support plate 4 having a depending peripheral flange 6 provided with slots 7 at intervals. In one embodiment of this cathode assembly satisfactory results have been secured when the cathode support plate 4 and peripheral flange, which together may be designated a cathode cup are fabricated from nickel sheet having a thickness of approximately 0.002 inch. Obviously, other appropriate thicknesses may be used. The slots 7 extend into the flange approximately 0.007 inch, and approximately 0.008 inch wide. For purposes of illustration, the diameter of the cathode in FIGS. 1, 2 and 4 has been illustrated greatly enlarged, the actual diameter of the cathode cup in one embodiment of the invention which has produced satisfactory results being approximately 0.052 inch. It should be understood that while the design of the cathode enables the fabrication of exceedingly small cathode assemblies, larger units may of course be made. To provide electron emissivity, the cathode cup is provided with an adherent layer 8 of triple carbonates as is usual with conventional cathodes.

The cathode cup with the layer 8 of emissive carbonates adherent thereon is associated as illustrated in FIGS. 1, 2 and 4, with a heater assembly designated generally by the numeral 9, and including a single filament 12 arranged in serpentine fashion in a single plane as illustrated in FIG. 3, the single filament 12 having a first terminus 13 and a second terminus 14 circumferentially spaced about 120 from the terminus 13, each terminus 13 and 14 being associated with a radially extending leg 16 and 17, respectively, as shown. A third leg 18 is provided which may form an integral part of the approximate midsection 19 of the filament. With respect to the leads 16, 17 and 18, as illustrated in FIG. 1, these heater leads extend radially outwardly from the heater and terminate in relatively large pads 19, 21 and 22, respectively. In an embodiment of this heater that has been successfully tested, the heater is fabricated from a tungsten/rhenium alloy foil, the heater foil being approximately 0.001 inch thick. Each of the legs 16, 17 and 18, is preferably approximately 0.003 inch wide, the legs being conveniently spaced at 120 intervals about a central axis extending through the center of the heater structure. As illustrated in FIG. 3, the heater leg 18 is provided with a wider portion 23 for purposes which will hereinafter be explained.

While a flat heater assembly has been fashioned from thin ribbon-like foil in the embodiment illustrated in FIG. 3, it is also contemplated that the heater assembly may be fabricated from a filament having a circular cross-section and arranged in a serpentine manner as illustrated in FIG. 9.

The heater thus formed is understandably fragile when constructed in the dimensions indicated. It is to be understood however that these dimensions are not intended to limit the invention, other dimensions being equally appropriate for different size cathode assemblies. However, it has been found advisable to protect the heater thus formed by supporting the central portion of the heater, i.e., that portion of the heater which embodies the serpentine arranged filament 12, between two wafers 24 and 26 of an appropriate electrical insulating material such as alumina ceramic. In a preferred embodiment, the wafer 24 is fabricated from beryllium oxide, this compound having a greater thermal conductivity than aluminum oxide. Where beryllia is used for the wafer 24, it is preferable that the wafer 26 utilize alumina which, compared to beryllia, is less thermally conductive.

The heater unit 9, sandwiched between the wafers 24 and 26, is placed within the confines of the cylindrical flange 6 of the cathode cup," the thicknesses of the wafers being gauged so as to just fill the cup so that the outersurface 27 of the wafer 26 lies substantially flush with the outer edge 28 of flange 6. When the assembly of heater unit and wafers 24 and 26 are properly positioned in the cathode cup, the heater legs l6, l7 and 18, being spaced at 120 intervals, project radially outwardly through the slots 7 formed in the flange 6, which are also spaced at 120 intervals. It will thus be seen that the upper surface 29 of the wafer 24 contiguously abuts the inner surface 31 of the cathode plate 4, thus heating this plate when the heater element is energized. Also, because of the greater thermal con ductivity of the wafer 24 than the wafer 26, the tendency is for heat from the heater filament to be conducted preferentially upwardly into the plate 4, with a lesser proportion of the heat being conducted downwardly through the wafer 26.

To rigidly retain the heater assembly within the cathode cup, retention plate 32 is disposed over the open end of the cathode cup, the retention plate in one embodiment being preferably formed from 0.0015 inch sheet nickel, and provided with a radially projecting integral tab 33 which is spot-welded or otherwise secured to the wide portion 23 of the heater lead 18. The remainder of the periphery of the retention plate 32 is appropriately spot-welded or otherwise secured to the edge 28 of the flange 6. Thus, heat passing downwardly through the wafer 26 passes into the retention plate which is formed from a metal having a relatively high coefficient of thermal-conductivity. The heat absorbed by this plate is conducted radially outwardly and into the flange 6 and significantly adds to the total heat energy absorbed by the plate 4. The retention plate and cathode plate are thus electrically connected to the heater lead 18 and are raised to whatever potential is impressed on the heater filament. If desired, the tab 33 may be omitted or shifted to a position in which it is electrically isolated from the heater filament and connected by another lead (not shown).

It is preferable that the wafers 24 and 26 which sandwich the heater filament 12 therebetween be formed so that they fill the spaces between the filament branches, thus insuring that each individual section or branch of the filament is surrounded by electrically insulative but thermally conductive material so as to increase the rigidity and non-destructability of the heater filament. One embodiment of this heater concept is illustrated in FIG. 5, in which the individual filament legs are shown to be rectangular. For additional convenience, and to relieve stress which might be generated in the wafers 24 and 26, the lateral edges 34 may be chamfered as illus trated in FIG. 6, to provide a knife edge 36 which lies in the plane along which the faces 38 and 37 of the wafers 24 and 26 abut. In the embodiment of the invention illustrated in FIG. 7, the lateral edges 34" of the heater filament 12 are undercut as illustrated to provide dovetails which form a positive lock with the surrounding ceramic wafer 24 and 26.

In each of the three different embodiments of the heater and sandwiching wafers 24 and 26, it will be understood that because that heater filament is fabricated in an extremely small size, the heater filament will be extremely fragile and derives considerable mechanical support from the enveloping wafers 24 and 26. Additionally, because of such fragility and because, as will hereinafter be explained, tension is applied to the heater legs 16 and 17 when the cathode package is mounted on a support structure, it is important that the terminations 13 and 14 be provided with projecting anchor means 39 and 41 respectively.

It has been found in practice that it is preferable that the ceramic wafers 24 and 26 be somewhat larger in diameter than the diameter of the filament package so that when the filament package is sandwiched between the Wafers 24 and 26, the ceramic material effectively embeds not only the intervening section of the filament but also the anchor portions 39 and 41. Thus, when tension is applied to the heater legs 16 and 17, there is a balanced resistance to such tension so as to prevent distortion of the heater filament. In like manner, and for similar reasons, the center tap formed by the attachment of the heater leg 18 to the center of the filament 12 is provided with shoulders 42 and 43 which lie embedded between the ceramic wafers 24 and 26 so that they effectively resist tension applied to the heater leg 18. In this regard, it is noted that the leg 18 may be detached from the heater as shown in FIG. 11, in which case the leg 18 would not function as an electrical center tap but only as a mechanical support.

SUPPORT STRUCTURE Referring to FIGS. 1, 2 and 8, there is provided for support of the cathode-heater assembly and for mounting purposes thereof in its ultimate environment, an annular ceramic ring 46, having an array of circumferentially regularly spaced apertures 47 thereabout, there being preferably twelve such apertures spaced at 30 intervals, adjacent the outer periphery 48 of the annular ceramic member 46. As illustrated in FIG. 1, the length of the heater legs l6, l7 and 18 are proportioned so that the tabs 19, 21 and 22, respectively, on the ends thereof, which are made relatively large to facilitate spotwelding, lie substantially centrally disposed over associated apertures 47 as shown.

Superimposed over the ceramic ring 46 at intervals as indicated in FIG. 1, are conductive lead members designated generally by the numerals 51, 52 and 53. The conductive lead member 51 comprises a radially extending leg 54 which lies beneath the outer extremity 82 of heater leg 16 and which terminates in a radiused end 55 coincident with the circular pad 19 formed on the end of the heater leg. Thus, both the pad 19 of the heater leg and the radiused end 55 of the terminal lead 54 terminate at a point centrally disposed with respect to the associated aperture 47. The temiinal leg 54 is integrally and electrically connected by an arcuate section 56 to a second radially extending lead 57 which terminates in a radiused end 58 centrally disposed in the aperture 47 next adjacent the heater leg 16. For reasons which will hereinafter be explained, the union of the terminal lead leg 54 and the arcuate connecting member 56 is provided with a bulge 59 as shown.

The terminal lead member 52 is similar to the terminal lead member 51 but is associated with the heater leg 17. In all other respects, the terminal lead 52 is identical to the terminal lead 51 and accordingly, corresponding primed reference numbers have been applied to this element of the combination.

The terminal lead structure designated by the numeral 53 in part corresponds to the terminal lead members 1 and 52, and insofar as the parts correspond, the same double primed reference numbers have been applied to this terminal lead member. However. this terminal lead member, in addition to having the arcuate connective band 56" connecting the lead member 54" and 57", is provided with a second arcuate connecting member 61 extending circumferentially from the terminal leg 57" in a direction opposite to the direction of the connector member 56". The arcuate connection member 61 is integral with the radially extending terminal lead 57" and, at its end remote from the terminal leg 57", the connection member 61 is integral with a terminal lead 62 having an outer end 63 terminating in a radiused end lying concentrically within the aperture 47 The terminal leg 62 is also provided at a point adjacent the innerperiphery 64 of the annular ceramic member 46 with a pair of circumferentially extending tabs 66 and 67. Beyond the tabs 66 and 67, the terminal lead 62 continues in a radially extending leg 68 of considerably smaller width than the portion 62, the thinned down radially extending leg portion 68 merging integrally with the outer periphery 69 of a reflector plate 71 closely underlying the cathode-heater assembly and functioning to reflect to the retention plate 32 any heat that is radiated therefrom in a downward direction.

To assist rigid support of the reflective plate 71, radially extending arms 72 and 73 spaced at 120 intervals from the leg 68 are also provided. The legs 72 and 73, as shown in FIG. 1, are provided at their outer ends with circumferentially extending tabs 66 and 67 similar to the tabs formed on the terminal lead 62. Preferably, the terminal lead members 51, 52, 53, 62 and legs 68, 72 and 73 are fabricated from sheet tungsten having a thickness of approximately 0.00l5 inch.

After superposition of the terminal lead members 51, 52 and 53 on the upper surface 74 of the annular ceramic member 46, there is superposed thereover a second ceramic ring 76 of smaller outside diameter, but having an inner diameter similar to the underlying ceramic ring 46. With the parts thus related, as seen in FIGS. 1, 2 and 8, the interconnecting members 56, 56 and 56", lie rigidly disposed between the two ceramic rings 46 and 76, and more specifically, the arcuate connection members or sections lie disposed between the inner and outer peripheries 77 and 78 of the smaller ceramic ring 76 as shown, thus precluding introduction of stress into support leads 16, I7 and 18 by attachment to 58 of electrical leads which form heater and cathode connections. Additionally, the circumferentially ex tending tabs 66 and 67 associated with legs 68, 72 and 73, also lie sandwiched between the annular ceramic members 46 and 76, and also lie disposed between the inner and outer peripheries of both ceramic members.

To secure the terminal lead members 51, 52 and 53, together with the legs 68, 72 and 73 rigidly to the ceramic members 46 and 76, the intervening space between the two annular ceramic members is filled with a suitable bonding material to form a layer 79 which is adherent to both opposing surfaces of the annular ceramic members 46 and 76 as shown best in FIG. 8. It has been found that a very rigid construction results from filling this space with a glass composition and then subjecting the assembly to sufficient heat to melt the glass and cause it to bond permanently to the opposed surfaces of the adjacent annular ceramic members. In some instances, it may be advisable to apply a limited amount of pressure on opposite sides of the stacked ceramic rings so as to insure that all of the metal surfaces lie tightly sandwiched between the adherent ceramic rings.

Upon completion of the adhesion of the annular support members 46 and 76 to each other, it will be seen that all of those metal parts that lie in the zone defined by the inner and outer peripheries of both ceramic rings will lie embedded and therefore locked in position between the two ceramic rings. They are thus rendered immovable in relation to each other and in relation to the supporting ceramic structure. It will also be seen that upon completion of the support sub-assembly the terminal leads 54, 54 and 54" will lie at intervals about the outer periphery of the ceramic 46, extending into concentric superposition over apertures 47 therein which are also positioned at l20 intervals.

CATHODE-HEATER-SUPPORT COMBINATION With the two sub-assemblies thus fabricated, it will be seen that the cathode sub-assembly including the heater portion thereof may be superimposed over the support subassembly so that the radially extending legs 16, 17 and 18 of the heater lie superimposed over the coincident with the underlying terminal leads 54, 54 and 54", respectively, as shown in FIG. 1. In this position of the parts, as illustrated in FIG. 2, the heater legs 16, 17 and 18 lie disposed on the top surface 81 of the annular ceramic member 76, and project radially beyond the outer periphery 77 thereof in portions 82, 83 and 84. To join the cathode-heater sub-assembly to the associated support sub-assembly, all that is required is that nickel tabs 86 be disposed between the heater leg pads 19, 21, 22 and the underlying end portion of associated terminal lead members 54, 54 and 54", respectively, whereupon the associated ends are brought together as illustrated in FIG. 2 and appropriately spotwelded to the intervening nickel pad 86.

As shown in FIG. 2, when the heater legs 16, 17 and 18 are brought together and spot-welded to the associated ends of the tungsten terminal leads 54, 54' and 54', the relatively wider tungsten lead members exert downward and radially outward components of force on the outboard portions of the heater legs that project beyond the outer periphery of the ceramic members 76, causing each heater leg to be resiliently biased downwardly as illustrated in FIG. 2. Such downwardly directed biasing force imposes a radially outwardly directed tensioning force on each heater leg, thus insuring that the cathode package remains centrally disposed with respect to the supporting annular ceramic structure. Additionally, it will be apparent that the cathode-heater package will be rigidly retained in position despite thermal cycling and the application of transversely directed impact shocks.

The planar cathode thus formed, is ready for mounting in the environment for which it was intended. For this purpose, selected ones of the apertures 47 may be used to receive posts or other supporting structure forming no part of this invention.

FIG. 9 EMBODIMENT While a flat ribbon-like filament has been illustrated in FIG. 3, it will of course be apparent that where appropriate a cylindrically formed filament such as illustrated in FIG. 9 may be arranged in serpentine fashion to provide leads 87 and 88 spaced at about 120 intervals. At the juncture of each lead with the body of the heater, the lead is formed to provide an anchor portion 89 adapted to be embedded between the ceramic wafers 24 and 26 in the same manner as the anchor portions 39 and 41 of the heater in FIG. 3. In this embodiment it is preferable that the third leg or lead 91 of the heater terminate in an anchor portion 92 that is embedded between the ceramic wafers 24 and 26 closely adjacent to the heater filament but not electrically connected to it, as shown in FIGv 9. The tab 33 on retention plate 32 may be secured to this lead to electrically isolate the cathode from the heater filament and to provide a separate lead to the cathode package. If an electrical connection is desired it may of course be made in any convenient manner.

FIG. EMBODIMENT J m flector be electrically isolated from the cathodeheater package. The embodiment illustrated in FIG. 10, accomplishes such isolation by eliminating the connecting member 61 so that the reflector plate lies suspended by the radially extending legs 68, 72 and 73 and their associated anchor tabs 66467. The terminal lead 62 may be used if desired to impress whatever potential is desired on the reflector plate 71.

In this embodiment, and in the embodiment illustrated in FIG. 1, it is desirable to protect the terminal leads at their outer extremities. To accomplish this purpose, as shown in FIGS. 10 and 11, while still permitting access to the terminal leads for electrical connection into an electrical circuit, the ceramic ring 76 is re placed by a ceramic ring 93 of larger diameter so that the outer periphery of the ring extends past the outer extremities of the terminal leads. For instance, the outer peripheries of both rings 93 and 46 may be coincident as shown in full lines, or the ring 93 may be somewhat greater in diameter as shown in broken lines in FIGS. 10 and 11. Since it is desirable to provide an edge 94 over which the heater legs 16, 17 and 18 may be resiliently displaced downwardly, the ring 93 is provided with apertures 95 that are relatively larger than apertures 47 in the underlying ring 46. For simplicity of assembly the number and spacing of the apertures 95 correspond to the apertures 47. Upon assembly, the apertures 95 are superimposed over the apertures 47 so that access may be had to the terminal extremities which after connection lie protected within the confines of the apertures 95, i.e., below the top surface thereof. In this respect it should be noted that proper positioning of the cathode in its ultimate environment is facilitated since the top surface 95' forms the reference plane for locating the cathode.

FIG. 12 EMBODIMENT While it has been discussed above that the heater leg 18 may be electrically disassociated from the heater filament even when the heater filament comprises a flat ribbon-like member, it is thought best to illustrate a configuration that has been found to be satisfactory. Accordingly, in FIG. 12, there is shown a flat ribbonlike filament 12 which in all respects may be similar to the filament illustrated in FIGS. 3, 5, 6 and 7, but which is not electrically connected to the leg 18. To anchor the end of the leg 18 next adjacent to the filament, an anchor tab 96 is provided adapted to be embedded between the two ceramic wafers 24 and 26 as previously discussed. As with the embodiment illustrated in FIG.

9, this embodiment lends itself to electrical isolation of the retention plate and therefore the cathode cup from the electrical potential imposed on the heater filament by the expedient of connection of the tab 33 to the electrically isolated leg 18.

Having thus described my invention, what is claimed to be novel and sought to be protected by Letters Patent, is as follows:

1. In combination, a cathode support structure and cathode-heater assembly supported thereon, comprisa. a pair of dielectric mounting rings arranged in endto-end relation and bonded to one another;

b. electrically conductive terminal means sandwiched between opposed ends of said mounting rings and including a plurality of resilient tensioning members; and

c. a cathode-heater assembly suspended on said mounting rings and including at least a pair of radially extending electrically conductive support legs electrically connected to said tensioning members so that a radially outwardly directed tensioning force is imposed on said support legs.

2. The combination according to claim 1, in which said mounting rings are annular and coaxially arranged about a longitudinal axis and said cathode-heater assembly is concentrically suspended in relation to said annular mounting rings.

3. The combination according to claim 1, in which said tensioning members extend radially outwardly beyond the outer periphery of one of said mounting rings.

4. The combination according to claim 1, in which said mounting rings have equal inside diameters and unequal outside diameters.

5. The combination according to claim 1, in which said electrically conductive support legs extend radially outwardly beyond the outer periphery of one of said mounting rings.

6. The combination according to claim 1, in which shield means are provided supported on said mounting rings and disposed adjacent said cathode-heater assembly on the side thereof remote from the cathode.

7. The combination according to claim 1, in which shield means are provided supported on said mounting rings and electrically connected to said electrically conductive support legs through said tensioning members.

8. The combination according to claim 1, in which said electrically conductive support legs are fabricated from a tungsten/rhenium alloy and said tensioning members are fabricated from tungsten, said support legs being resiliently biased toward said tensioning members and being tensioned thereby.

9. The combination according to claim 1, in which said support legs are fabricated from a tungsten/- rhenium alloy, said tensioning members are fabricated from tungsten, and nickel pad means are disposed between the terminal ends of said support legs and said tensioning members to which both may be spotwelded to form a continuous electrical circuit therethrough.

10. The combination according to claim 1, in which said mounting rings are provided with a multiplicity of circumferentially spaced apertures, and the terminal ends of said support legs and tensioning members terminate in selected ones of said apertures.

l 1. The combination according to claim 1, in which bonding means are provided between opposed surfaces of said mounting rings to bind the rings and the interposed electrically conductive terminal means sandwiched therebetween into a composite structure.

12. The combination according to claim 1, in which said cathode-heater assembly includes a cathode plate portion and a flange portion integral therewith, a layer of electron emissive material adherent to the plate portion of said cathode on the side thereof remote from said flange, and an electrically energizable filament constituting a cathode heater unit disposed within said flange parallel with said plate portion and said layer of electron emissive material.

13. The combination according to claim 1, in which said cathode-heater assembly includes three electrically conductive support legs electrically connected to three tensioning members, said support legs and tensioning members being spaced 120 apart.

14. The combination according to claim 1, in which said cathode-heater assembly includes a cathode having a plate portion integral with a peripheral flange portion, said flange portion being provided with a plurality of circumferentially spaced slots therein, a layer of electron emissive material adherent to the surface of said plate portion remote from said peripheral flange, and an electrically energizable filament constituting a cathode heater unit disposed within the confines of said peripheral flange, said support legs being integrally connected to said heater element and projecting radially outwardly therefrom through the slots in said peripheral flange, electrically insulative thermally conductive means surrounding the heater unit within said cathode flange, and a closure plate connected to the peripheral flange on the end thereof remote from said plate portion on which said layer of electron emissive material is adherent to retain said heater unit within said flange and to conduct heat from said heater unit to said peripheral flange to increase the efficiency of said cathode-heater assembly.

15. The combination according to claim 1, in which said cathode-heater assembly includes an electrically energizable planar heater unit comprising a filament formed to provide a plurality of turns arranged in serpentine fashion with all of the turns of the filament lying in a common plane, a pair of electrically conductive support legs connected to selected portions of said filament and functioning also as electrically conductive terminal leads for said heater unit, a pair of dielectric wafers disposed on opposite sides of said planar heater unit and embedding the turns of said filament therebetween to lend physical support thereto and provide electrical insulation therebetween, a third support leg disposed between said dielectric wafers and anchored thereby, said first mentioned pair of support legs and said third support leg extending radially away from the heater unit in the same plane thereof, a cathode including a plate portion disposed in planar parallelism to said heater unit and having one surface thereof contiguous to one of said dielectric wafers, said cathode including a peripheral flange integral with said plate portion and enveloping the outer periphery of said dielectric wafers, and a layer of electron emissive material adherent to the surface of said cathode plate portion on the side thereof remote from the surface to which said dielectric wafer is contiguous.

16. The combination according to claim 1, in which said electrically conductive support legs are of uniform width over a major portion of their length and are provided at their extreme ends with a substantially wider integral portion to facilitate spotwelding of said relatively larger end portion to an associated tensioning member.

17. The combination according to claim 1, in which said cathode-heater assembly includes a heater unit formed from a single continuous filament having a flat ribbon-like cross-section and formed to provide a plurality of turns arranged in serpentine fashion in a single plane to produce a planar heater unit, said planar heater unit being provided with at least two radially extending support legs spaced thereabout.

18. The combination according to claim 1, in which said cathode-heater assembly includes a heater unit fabricated from a wire-like filament cylindrical in cross-section and formed to provide a plurality of turns arranged in serpentine fashion in a single plane, the electrically conductive support legs of said pair thereof being integral with opposite end portions of said filament and extending radially away therefrom to support said filament in selected relation to said mounting rings.

19. The combination according to claim 1, in which said electrically conductive support legs extend radially outwardly from said cathode-heater assembly in superposed relation to one of said mounting rings, said one of said mounting rings being provided with a relatively sharp discontinuity at a point spaced from the inner periphery thereof over and beyond which said legs project, said tensioning members being disposed on the opposite side of said one of said mounting rings whereby when said support legs and said tensioning members are spot-welded together at their extreme ends, the support legs are resiliently biased to impose tension said support legs.

20. The combination according to claim 1, in which said pair of mounting rings are annular and coaxially arranged about a longitudinal axis and each mounting ring is provided with a multiplicity of apertures therethrough circumferentially spaced thereabout adjacent the outer periphery thereof, said mounting rings being oriented one to the other so that the apertures in the rings register, said support legs and said tensioning members terminating opposite selected ones of said registering apertures whereby access may be had to the associated ends of said support legs and tensioning members through said apertures.

21. The combination according to claim 1, in which said mounting rings are annular and coaxially arranged about a longitudinal axis, said mounting rings each being provided with a plurality of circumferentially spaced apertures, said mounting rings being associated one with the other whereby said apertures in opposed mounting rings register one with another, said support legs projecting radially outwardly across one surface of one of said mounting rings and terminating opposite selected ones of the apertures formed in said mounting ring while the associated tension members extend radially across the opposed surface of said mounting ring and terminate opposite the same selected ones of the apertures whereby when the ends of said support legs are spotwelded to the ends of said tensioning members the end portions of said support legs are biased into said apertures below the top surface of said mounting ring.

22. The combination according to claim 1, in which said mounting rings are annular and coaxially arranged about a longitudinal axis, one of said mounting rings being of larger diameter than the other and having a top surface forming a reference plane across which said support legs extend and on which said support legs are supported, both said mounting rings having a plurality of circumferentially spaced apertures, said mounting rings being oriented so that said apertures register one with another, said electrically conductive terminal means including an electrically conductive connector portion embedded between said mounting rings and integral at one end with an associated tensioning member extending radially therefrom and terminating opposite one of said apertures, and integral at its other end with a radially extending terminal member terminating opposite the aperture next adjacent to the aperture opposite which the tensioning member terminates whereby electrical connection may be made to said terminal members without imposing mechanical or thermal stresses on the associated tensioning members and said support legs.

23. The combination according to claim 22, in which shield means are provided supported on said mounting rings and disposed adjacent said cathode-heater assembly on the side thereof remote from the cathode, and suspension means integral with said shield and including a plurality of radially extending suspension members having their inner ends integral with the outer periphery of said shield and having their outer end portions sandwiched between said mounting rings.

24. A cathode-heater assembly including a heater unit including a filament formed to provide a plurality of turns arranged in serpentine fashion, electrical terminal leads connected to said filament at its outer periphery and constituting support legs therefor, a pair of dielectric wafers sandwiching said heater unit to support the individual turns of said filament against relative displacement, a cathode plate disposed across and contiguous to one of said wafers, and a layer of electron emissive material adherent on said cathode plate on the side thereof opposite said dielectric wafers.

25. The combination according to claim 24, in which means are provided retaining said heater unit including one of said dielectric wafers contiguous to said cathode plate on the side thereof remote from said electron emissive material.

26. The combination according to claim 24, in which said heater unit lies embedded between said dielectric wafers.

27. The combination according to claim 24, in which said cathode plate is provided with a peripheral flange having slots therein and said heater unit terminal leads and support legs extend radially through said slots.

28. The combination according to claim 24, in which a retention plate is thermally connected to said cathode plate to retain said heater unit including said dielectric wafers next adjacent said cathode plate to thermally conduct heat from said heater unit to said cathode plate.

29. The combination according to claim 24, in which said filament is rectangular in cross section.

30. The combination according to claim 24, in which said filament is generally rectangular in cross section and is provided with chamfered edges which terminate in a plane between said dielectric wafers.

31. The combination according to claim 24, in which said filament is generally rectangular in configuration, the lateral edges of said filament being undercut to provide a dovetailed cross section.

32. The combination according to claim 24, in which said filament is circular in cross section.

33. The combination according to claim 24, in which said dielectric wafers are on the order of 0.005 inch in thickness and said filament is on the order of 0.001

inch in thickness.

34. The combination according to claim 24, in which said heater unit comprises a single continuous filament, and said plurality of turns arranged in a serpentine array all lie in a common plane.

35. The combination according to claim 24, in which said cathode plate lies parallel to one of said dielectric wafers and the associated heater filament.

36. The combination according to claim 24, in which three support legs are provided for said heater unit, at least two of said support legs constituting terminal leads for said heater filament.

37. The combination according to claim 25, in which said retention means comprises a metal plate having a high coefficient of thermal conductivity thermally connected to said cathode plate.

38. The combination according to claim 25, in which said retention means is electrically connected to said heater filament.

39. The combination according to claim 25, in which said retention means is electrically connected to said cathode plate.

40. The combination according to claim 25, in which said retention means is electrically connected to said heater filament and to said cathode plate.

41. The combination according to claim 25, in which said retention means is thermally connected to said cathode plate.

42. The combination according to claim 25, in which said retention means is electrically and thermally connected to said cathode plate.

43. The combination according to claim 25, in which said retention means is electrically connected to said heater filament and said cathode plate, and thermally connected to said cathode plate.

44. The combination according to claim 27, in which a retention plate is thermally connected to said flang on the end thereof remote from said cathode plate 0 retain said heater unit within the flange and conduct heat from the heater unit to the cathode plate.

45. In combination, a cathode support structure and cathode-heater assembly supported thereon, comprising:

a. a dielectric mounting ring arranged about a longitudinal axis and having a transverse dimension larger than its thickness measured in the axial direction;

at least two electrically conductive terminal means mounted on one end of said ring and including a plurality of tensioning members; and c. a cathode-heater assembly mounted on the ring and including at least two electrically conductive support legs attached to said heater and extending across the opposite end of said ring from said electrically conductive terminal means and electrically connected to said tensioning members.

46. The combination according to claim 45, in which said tensioning members and said support legs extend transversely in relation to said mounting ring.

47. The combination according to claim 45, in which said tensioning members are resiliently biased in an axial direction to impose a radially directed component of force on said support legs to retain the cathodeheater-assembly immovable in relation to the mounting ring.

48. The combination according to claim 45, in which each said electrically conductive terminal means comprises a generally U-shaped member, one leg of the U- shaped member constituting a terminal lead and the other leg of the U-shaped member constituting a tensioning member.

49. The combination according to claim 45, in which each said electrically conductive terminal means comprises a generally U-shaped member having the portion of the U-shaped member connecting the legs mounted on the ring and the leg portions of the Ushaped member extending transversely in relation to the ring beyond a discontinuity therein, one of the legs of each U- shaped member constituting a tensioning member, whereby said support legs may be connected to said tensioning members beyond said discontinuity.

50. The combination according to claim 45, in which each said electrically conductive terminal means comprises a generally U-shaped member, one leg of the U- shaped member constituting a terminal lead and the other leg of the U-shaped member constituting a tensioning member, each said tensioning member being resiliently biased out of the plane of said terminal lead upon attachment to an associated support leg.

51. The combination according to claim 45, in which shield means are provided supported on the same end of said ring as said terminal means and disposed adjacent said cathode-heater assembly on the side thereof remote from said cathode.

52. The combination according to claim 45, in which said cathode-heater assembly comprises a heater unit including a filament formed to provide a plurality of turns arranged in serpentine fashion, said electrically conductive support legs constituting terminal leads for said heater filament, a pair of dielectric wafers between which said filament is sandwiched; and a cathode plate having a layer of electron emissive material thereon disposed across and contiguous to one of said wafers.

53. The combination according to claim 45, in which three support legs are provided for said cathode-heater minal leads for said heater unit. 

1. In combination, a cathode support structure and cathodeheater assembly supported thereon, comprising: a. a pair of dielectric mounting rings arranged in end-to-end relation and bonded to one another; b. electrically conductive terminal means sandwiched between opposed ends of said mounting rings and including a plurality of resilient tensioning members; and c. a cathode-heater assembly suspended on said mounting rings and including at least a pair of radially extending electrically conductive support legs electrically connected to said tensioning members so that a radially outwardly directed tensioning force is imposed on said support legs.
 2. The combination according to claim 1, in which said mounting rings are annular and coaxially arranged about a longitudinal axis and said cathode-heater assembly is concentrically suspended in relation to said annular mounting rings.
 3. The combination according to claim 1, in which said tensioning members extend radially outwardly beyond the outer periphery of one of said mounting rings.
 4. The combination according to claim 1, in which said mounting rings have equal inside diameters and unequal outside diameters.
 5. The combination according to claim 1, in which said electrically conductive support legs extend radially outwardly beyond the outer periphery of one of said mounting rings.
 6. The combination according to claim 1, in which shield means are provided supported on said mounting rings and disposed adjacent said cathode-heater assembly on the side thereof remote from the cathode.
 7. The combination according to claim 1, in which shield means are provided supported on said mounting rings and electrically connected to said electrically conductive support legs through said tensioning members.
 8. The combination according to claim 1, in which said electrically conductive support legs are fabricated from a tungsten/rhenium alloy and said tensioning members are fabricated from tungsten, said support legs being resiliently biased toward said tensioning members and being tensioned thereby.
 9. The combination according to claim 1, in which said support legs are fabricated from a tungsten/rhenium alloy, said tensioning members are fabricated from tungsten, and nickel pad means are disposed between the terminal ends of said support legs and said tensioning members to which both may be spotwelded to form a continuous electrical circuit therethrough.
 10. The combination according to claim 1, in whIch said mounting rings are provided with a multiplicity of circumferentially spaced apertures, and the terminal ends of said support legs and tensioning members terminate in selected ones of said apertures.
 11. The combination according to claim 1, in which bonding means are provided between opposed surfaces of said mounting rings to bind the rings and the interposed electrically conductive terminal means sandwiched therebetween into a composite structure.
 12. The combination according to claim 1, in which said cathode-heater assembly includes a cathode plate portion and a flange portion integral therewith, a layer of electron emissive material adherent to the plate portion of said cathode on the side thereof remote from said flange, and an electrically energizable filament constituting a cathode heater unit disposed within said flange parallel with said plate portion and said layer of electron emissive material.
 13. The combination according to claim 1, in which said cathode-heater assembly includes three electrically conductive support legs electrically connected to three tensioning members, said support legs and tensioning members being spaced 120* apart.
 14. The combination according to claim 1, in which said cathode-heater assembly includes a cathode having a plate portion integral with a peripheral flange portion, said flange portion being provided with a plurality of circumferentially spaced slots therein, a layer of electron emissive material adherent to the surface of said plate portion remote from said peripheral flange, and an electrically energizable filament constituting a cathode heater unit disposed within the confines of said peripheral flange, said support legs being integrally connected to said heater element and projecting radially outwardly therefrom through the slots in said peripheral flange, electrically insulative thermally conductive means surrounding the heater unit within said cathode flange, and a closure plate connected to the peripheral flange on the end thereof remote from said plate portion on which said layer of electron emissive material is adherent to retain said heater unit within said flange and to conduct heat from said heater unit to said peripheral flange to increase the efficiency of said cathode-heater assembly.
 15. The combination according to claim 1, in which said cathode-heater assembly includes an electrically energizable planar heater unit comprising a filament formed to provide a plurality of turns arranged in serpentine fashion with all of the turns of the filament lying in a common plane, a pair of electrically conductive support legs connected to selected portions of said filament and functioning also as electrically conductive terminal leads for said heater unit, a pair of dielectric wafers disposed on opposite sides of said planar heater unit and embedding the turns of said filament therebetween to lend physical support thereto and provide electrical insulation therebetween, a third support leg disposed between said dielectric wafers and anchored thereby, said first mentioned pair of support legs and said third support leg extending radially away from the heater unit in the same plane thereof, a cathode including a plate portion disposed in planar parallelism to said heater unit and having one surface thereof contiguous to one of said dielectric wafers, said cathode including a peripheral flange integral with said plate portion and enveloping the outer periphery of said dielectric wafers, and a layer of electron emissive material adherent to the surface of said cathode plate portion on the side thereof remote from the surface to which said dielectric wafer is contiguous.
 16. The combination according to claim 1, in which said electrically conductive support legs are of uniform width over a major portion of their length and are provided at their extreme ends with a substantially wider integral portion to facilitate spotwelding of said relatively larger end portion to an associated tensioning mEmber.
 17. The combination according to claim 1, in which said cathode-heater assembly includes a heater unit formed from a single continuous filament having a flat ribbon-like cross-section and formed to provide a plurality of turns arranged in serpentine fashion in a single plane to produce a planar heater unit, said planar heater unit being provided with at least two radially extending support legs spaced thereabout.
 18. The combination according to claim 1, in which said cathode-heater assembly includes a heater unit fabricated from a wire-like filament cylindrical in cross-section and formed to provide a plurality of turns arranged in serpentine fashion in a single plane, the electrically conductive support legs of said pair thereof being integral with opposite end portions of said filament and extending radially away therefrom to support said filament in selected relation to said mounting rings.
 19. The combination according to claim 1, in which said electrically conductive support legs extend radially outwardly from said cathode-heater assembly in superposed relation to one of said mounting rings, said one of said mounting rings being provided with a relatively sharp discontinuity at a point spaced from the inner periphery thereof over and beyond which said legs project, said tensioning members being disposed on the opposite side of said one of said mounting rings whereby when said support legs and said tensioning members are spot-welded together at their extreme ends, the support legs are resiliently biased to impose tension said support legs.
 20. The combination according to claim 1, in which said pair of mounting rings are annular and coaxially arranged about a longitudinal axis and each mounting ring is provided with a multiplicity of apertures therethrough circumferentially spaced thereabout adjacent the outer periphery thereof, said mounting rings being oriented one to the other so that the apertures in the rings register, said support legs and said tensioning members terminating opposite selected ones of said registering apertures whereby access may be had to the associated ends of said support legs and tensioning members through said apertures.
 21. The combination according to claim 1, in which said mounting rings are annular and coaxially arranged about a longitudinal axis, said mounting rings each being provided with a plurality of circumferentially spaced apertures, said mounting rings being associated one with the other whereby said apertures in opposed mounting rings register one with another, said support legs projecting radially outwardly across one surface of one of said mounting rings and terminating opposite selected ones of the apertures formed in said mounting ring while the associated tension members extend radially across the opposed surface of said mounting ring and terminate opposite the same selected ones of the apertures whereby when the ends of said support legs are spotwelded to the ends of said tensioning members the end portions of said support legs are biased into said apertures below the top surface of said mounting ring.
 22. The combination according to claim 1, in which said mounting rings are annular and coaxially arranged about a longitudinal axis, one of said mounting rings being of larger diameter than the other and having a top surface forming a reference plane across which said support legs extend and on which said support legs are supported, both said mounting rings having a plurality of circumferentially spaced apertures, said mounting rings being oriented so that said apertures register one with another, said electrically conductive terminal means including an electrically conductive connector portion embedded between said mounting rings and integral at one end with an associated tensioning member extending radially therefrom and terminating opposite one of said apertures, and integral at its other end with a radially extending terminal member terminating opposite the aperture next adjacent to the aperture oPposite which the tensioning member terminates whereby electrical connection may be made to said terminal members without imposing mechanical or thermal stresses on the associated tensioning members and said support legs.
 23. The combination according to claim 22, in which shield means are provided supported on said mounting rings and disposed adjacent said cathode-heater assembly on the side thereof remote from the cathode, and suspension means integral with said shield and including a plurality of radially extending suspension members having their inner ends integral with the outer periphery of said shield and having their outer end portions sandwiched between said mounting rings.
 24. A cathode-heater assembly including a heater unit including a filament formed to provide a plurality of turns arranged in serpentine fashion, electrical terminal leads connected to said filament at its outer periphery and constituting support legs therefor, a pair of dielectric wafers sandwiching said heater unit to support the individual turns of said filament against relative displacement, a cathode plate disposed across and contiguous to one of said wafers, and a layer of electron emissive material adherent on said cathode plate on the side thereof opposite said dielectric wafers.
 25. The combination according to claim 24, in which means are provided retaining said heater unit including one of said dielectric wafers contiguous to said cathode plate on the side thereof remote from said electron emissive material.
 26. The combination according to claim 24, in which said heater unit lies embedded between said dielectric wafers.
 27. The combination according to claim 24, in which said cathode plate is provided with a peripheral flange having slots therein and said heater unit terminal leads and support legs extend radially through said slots.
 28. The combination according to claim 24, in which a retention plate is thermally connected to said cathode plate to retain said heater unit including said dielectric wafers next adjacent said cathode plate to thermally conduct heat from said heater unit to said cathode plate.
 29. The combination according to claim 24, in which said filament is rectangular in cross section.
 30. The combination according to claim 24, in which said filament is generally rectangular in cross section and is provided with chamfered edges which terminate in a plane between said dielectric wafers.
 31. The combination according to claim 24, in which said filament is generally rectangular in configuration, the lateral edges of said filament being undercut to provide a dovetailed cross section.
 32. The combination according to claim 24, in which said filament is circular in cross section.
 33. The combination according to claim 24, in which said dielectric wafers are on the order of 0.005 inch in thickness and said filament is on the order of 0.001 inch in thickness.
 34. The combination according to claim 24, in which said heater unit comprises a single continuous filament, and said plurality of turns arranged in a serpentine array all lie in a common plane.
 35. The combination according to claim 24, in which said cathode plate lies parallel to one of said dielectric wafers and the associated heater filament.
 36. The combination according to claim 24, in which three support legs are provided for said heater unit, at least two of said support legs constituting terminal leads for said heater filament.
 37. The combination according to claim 25, in which said retention means comprises a metal plate having a high coefficient of thermal conductivity thermally connected to said cathode plate.
 38. The combination according to claim 25, in which said retention means is electrically connected to said heater filament.
 39. The combination according to claim 25, in which said retention means is electrically connected to said cathode plate.
 40. The combination according to claim 25, in which said retention means is electricAlly connected to said heater filament and to said cathode plate.
 41. The combination according to claim 25, in which said retention means is thermally connected to said cathode plate.
 42. The combination according to claim 25, in which said retention means is electrically and thermally connected to said cathode plate.
 43. The combination according to claim 25, in which said retention means is electrically connected to said heater filament and said cathode plate, and thermally connected to said cathode plate.
 44. The combination according to claim 27, in which a retention plate is thermally connected to said flange on the end thereof remote from said cathode plate to retain said heater unit within the flange and conduct heat from the heater unit to the cathode plate.
 45. In combination, a cathode support structure and cathode-heater assembly supported thereon, comprising: a. a dielectric mounting ring arranged about a longitudinal axis and having a transverse dimension larger than its thickness measured in the axial direction; b. at least two electrically conductive terminal means mounted on one end of said ring and including a plurality of tensioning members; and c. a cathode-heater assembly mounted on the ring and including at least two electrically conductive support legs attached to said heater and extending across the opposite end of said ring from said electrically conductive terminal means and electrically connected to said tensioning members.
 46. The combination according to claim 45, in which said tensioning members and said support legs extend transversely in relation to said mounting ring.
 47. The combination according to claim 45, in which said tensioning members are resiliently biased in an axial direction to impose a radially directed component of force on said support legs to retain the cathode-heater-assembly immovable in relation to the mounting ring.
 48. The combination according to claim 45, in which each said electrically conductive terminal means comprises a generally U-shaped member, one leg of the U-shaped member constituting a terminal lead and the other leg of the U-shaped member constituting a tensioning member.
 49. The combination according to claim 45, in which each said electrically conductive terminal means comprises a generally U-shaped member having the portion of the U-shaped member connecting the legs mounted on the ring and the leg portions of the U-shaped member extending transversely in relation to the ring beyond a discontinuity therein, one of the legs of each U-shaped member constituting a tensioning member, whereby said support legs may be connected to said tensioning members beyond said discontinuity.
 50. The combination according to claim 45, in which each said electrically conductive terminal means comprises a generally U-shaped member, one leg of the U-shaped member constituting a terminal lead and the other leg of the U-shaped member constituting a tensioning member, each said tensioning member being resiliently biased out of the plane of said terminal lead upon attachment to an associated support leg.
 51. The combination according to claim 45, in which shield means are provided supported on the same end of said ring as said terminal means and disposed adjacent said cathode-heater assembly on the side thereof remote from said cathode.
 52. The combination according to claim 45, in which said cathode-heater assembly comprises a heater unit including a filament formed to provide a plurality of turns arranged in serpentine fashion, said electrically conductive support legs constituting terminal leads for said heater filament, a pair of dielectric wafers between which said filament is sandwiched; and a cathode plate having a layer of electron emissive material thereon disposed across and contiguous to one of said wafers.
 53. The combination according to claim 45, in which three support legs are provided for said cathode-heater assembly, at least two of whIch constitute electrical terminal leads for said heater unit. 